At some point, the chances are strong that you will need to mount photopolymer plate for printing. Zinc etched plates and brass plates are less and less common as photopolymer becomes more and more affordable and accessible. To this end a review of the options is in order.
You can just bang your plate onto type high wood with a few small nails, but ply is often too rough to give a consistent result if the plate area is large, MDF is usable but nasty to work with and unless you have a thicknesser of some sort it can be tricky getting it to type height (a problem no matter which wood you use). The other issue is that plate mounted onto wood remains on that wood, as removal tends to ruin the plate (even if you are really careful there is often slight creasing and such).
The other options are type high mounting blocks often made from aluminium sheet which is covered or inlaid with magnets. Bases of this sort can also provide extra advantages as a ruled grid can be put onto the base for ease of registration.
I suspect that the flat magnetic bases are best for proofing presses and the boxcar type bases better for platens. Having said that the Bunting type magnetic base is the base I hear most often reccomended for Heidelberg windmills.
I'll examine the three main plate types here starting with the most readily available and the cheapest which is the boxcar base.
Boxcar press are as dedicated to private & indie presses as anyone could ever be. They are also the first point of call for many beginning letterpress printers, and are regarded highly by many of the experienced private press printers I've spoken with.
Boxcar bases, are a non magnetised gridded aluminium base for use with plastic based photopolymer plates.
The plastic backed photopolymer from Boxcar has a transparent back which assists greatly with plates registration for multi colour jobs.
Boxcar Bases are easily the cheapest option followed closely by Patmag
Boxcar bases require that the polymer be adhered to the surface of the plate using double sided tape (or possibly glue). The use of tape can be wasteful and expensive and can create problems with plate mounting and removal. To use enough tape to cover the block evenly and not create inking inconsistencies you must be using enough tape to make plate preservation upon removal at least a bit of an issue. I'd like to see how easy the plate removal is after its been put through a platen press for a few hundred impressions.
Patmag bases are a slab of aluminium cut to size and height with an overlay of magnetic material on top.
They work well, they are affordable and they will get you printing quickly. I found Patrick Reagh excellent to deal with (which is not always the way of it with larger companies although my dealings with Boxcar Press and Bunting Magnetics have also been good).
Given the propensity for magnets to lose magnetism when dropped, banged or compressed, I'm curious about how long they might last. If they did fail then using the blocks in a demagnetised state would be possible. They would then be pretty much the same as a boxcar base without the grid.
If you do deep relief impressions I'm not sure how they would hold up over a long period of time
Drawing a grid on the magnetic material for registration is not great due to the colour of the magnetic material but using small steel rulers is a real treat for registration place holders.
The Bunting plate (Magnetic Cerface™ Flat Base) is the top of the range, an aluminium plate, with magnets inserted into the body side to side which forces a magnetic field around the whole block. This causes the plate to suck to the bed of the press and to pull the steel back photopolmer plate into a strong embrace.
Solid, powerful and reliable not much chance of it wearing out anytime soon.
An A4 sized plate will cost you around $1000USD at the time of writing this (2011), it is very much the plate for commercial printers at that price. This really is the block of choice, but the price is prohibitive for many small presses. The plate is powerful enough that care in handling is essential.