Hi Jeremy,
I agree with everyone's suggestion of using the old base as a template and just copying it for your new base.
The only other thing I would suggest is using Baltic Birch Ply instead of Marine Ply. Baltic Birch Ply uses Hardwood (Usually Silver Birch or similar) on all the internal cores and both faces and uses Waterproof A Bond (Formaldehyde) Glue. It is what just about all of the professional telescope builders use including Peter Read at SDM and Dave Kreige at Obsession. Marine Ply uses softwood (often Hoop Pine) on all cores and both faces and also uses Waterproof A Bond (Formaldehyde) Glue. While Marine Ply offers the ultimate weatherproofing, it isn't physically as durable, or as strong, as Baltic Birch Ply and is prone to dings and nicks, being of all softwood construction. 18mm Baltic Birch Ply uses 13 x Hardwood Laminations, whereas 18mm Marine Ply uses 9 x Softwood Laminations.
If you were taking the telescope swimming I would suggest marine ply. On the basis you will be using it on land, I would suggest Baltic Birch Ply each and every time. You can then finish it off with a stain and polyurethane clear, or use Marine Varnish. Baltic Birch Ply is more weather resistant than you will ever need for a telescope. If it wasn't the best choice, the professional telescope builders wouldn't be using it, they would be using marine ply.
18mm Baltic Birch Ply $132 per sheet
18mm Marine Ply $132 per sheet
Cheers
John B