For the platen I had in mind at each end of the ‘V’ shape a triangular block with an included angle of 100º, with the acrylic screwed through two holes drilled in each side. Screwing through the sheet into pilot holes in the block seems better than trying to screw into holes in thin acrylic, as David Landin did.
The plastic board I have has a rigid foam core which takes screws well, and is around 0.70” thick so slightly thinner than 3/4”. If the triangular pieces were around two or three inches high when inserted in the platen 'V' they would be very light and should provide secure mounting for the acrylic sheets, probably better than the small plastic blocks that David Landin suggested, and possibly better than the small metal brackets illustrated, which would have to be bent out to 100º. The plastic board offcuts I have are from boards widely used here for fascia and similar boards on houses. I don’t know if similar material is used in the U.S., but that is probably academic as it is unlikely that
dpc has any offcuts.
I have been thinking about the best way to cut some blocks using my power mitre or circular saw, given the the need for left and right cuts, and the need to be able to hold the board for the second cut without wasting too much strip length. It has only been an intellectual exercise so far, as I like to use my circular saw and mitre saw outside, and there have been some very cold winds recently.
Something similar could, of course, be made from lumber sheet, maybe MDF if it will drill and take screws reliably, or plywood, or maybe hardwood sheet would be ideal as it would probably have a finer grain. Other designs are of course possible, including designs that could allow the acrylic sheets to be removed for storage, but simplicity and low weight are probably both desirable.
For the 100º book cradle, I have already suggested a possible simple solution using standard 90º metal brackets, but I’m sure that
dpc can see ways of cutting suitable mountings for the MDF sheets which could be screwed to the sliding baseboard, from whatever board he has available, if he can see any advantage in doing that.
With a 90º platen angle the two perspex sheets butt neatly against each other at the bottom of the ‘V’, which David Landin mentioned was ideal when the platen was pressed into the centre of the opened book. As the following illustration shows, that isn’t quite the case for a 100º platen. Ideally the bottom edges of the acrylic sheets would be cut at a slight angle so that they could butt together, but while it might be possible that probably isn’t a realistic option, and
dpc has mentioned that placing something under the centre of the book would assist in inserting the platen into the book.
Any thoughts on those ideas?