A daggerboard case cover looks a bit like a short daggerboard and prevents water sloshing up, and out of, the daggerboard case when the boat is under outboard motor or being towed. At the time of writing, you can buy them (e.g. from TridentUK). This article give some basic dimensions for those wanting to make your own. Needless to say, you need to check the actual measurements on your boat to get a more precise and better fit.
Width of daggerboard slot, according to the rules (1.4.11), is between 11mm and 19mm. In practice, Winders have the narrowest slots at 13mm. Most are between 14 and 19mm (max daggerboard thickness is 14mm). The "standard" ply daggerboard (i.e. the one supplied with a kit) was 12mm thick, so if you go for 12mm ply for your case cover, you should be fine.
Length of slot is 381 to 405mm.
One of the reviews of the TridentUK cover https://www.tridentuk.com/gb/product-mirror-daggerboard-case-cover-trmirdcc.html#Reviews suggests the ply should go all the way to the bottom of the daggerboard case. The mimimum daggerboard case depth is 283mm, so this, plus the thickness of the piece of light coloured wood (see below), gives the final dimension of the piece of plywood, around 283 + 15 = 298mm.
No class rule regarding the slot in the thwart, but if I measure a "standard" ply daggerboard, the handles finish 105mm ahead of the aft edge (i.e. the distance the from the end of that piece of light coloured wood to the edge of the ply on the Trident photo). The thwart is between 13mm and 17mm thick (Wooden boat Specification states 15mm), so that gives you the thickness of that piece of light coloured wood in the Trident photo.
Daggerboard case top battens are 20mm X 16mm according to the wooden boat spec, so assuming these are fitted with widest surface horizontal, we can calculate the width of daggerboard case and capping is about 16mm (slot) + 2 X 5mm (ply sides) + 2 X 20mm (battens) = 16 + 10 + 40 mm = 66mm so this would be the width of the top piece of wood.
The daggerboard case side (i.e. front & back vertical) battens are 25mm X 16mm, so your top piece of wood needs to overhang the plywood at the forward end by about 25mm (like the Trident one does).
