Paul Cleaver's description of how to do this in the Tech Library is an excellent read. I don't remember whether his is a MkI or MkII Eclipse, but there is no difference in the design of that part of the boat in the two variants that I'm aware of. There are differences in the way pipes, drainholes and other assorted build features are arranged between individual boats in each of the variants so improvising on Paul's design and layout is likely to be needed. The Tek-Tanks' E33 holding tank design is (as I understand it) Paul's design which they now sell to other owners.
The only other place I can think of that is realistically useful for placing a holding tank is under the port side of the v-berth, though routing pipework from the head, back to the seacock (if required) and for the pump out and vent pipes would be a lot more challenging unless a corner of the chain locker is used. I'd not want the tank in the cabin, nor pipework of the length needed, nor pipework in the chain locker where it could be damaged so to my mind Paul's solution of placing the tank behind the bathroom cabinet is the only good solution.
As for the size, Paul's description mentions a (possibly superceded) Spanish specification that should be searchable on the internet that requires a minumum 40 litre tank for a vessel the size of the Eclipse so the 53 litre tank gives ample room. It might be possible to fit a larger tank, especially if you don't want the gravity drain option Paul specified, but you'd likely find it necessary to cut the vanity unit surface or something else in order to manouvre it into position. Paul's design is effectively invisible once fitted and the panels reinstalled. You'd also have to specify the design yourself.
I feel that the membership cost would be justified for accessing this one resource in itself, let alone the vast amount of other information the site has to offer.
Best wishes,
Paul
