With a group, where a half dozen pairs will be wound the additional cost of an original( of what ever OPT is selected ) is not too bad. Some of the likely suspects would be the Acro TO-300, a 6k6 20W item, any of the 20W/5k Peerless 20-20 designs or OEM variants

For a PP 2A3 amp, operating at a SE-specified point( 2k5-5k SE load), anywhere between 5 and 10k a-a ought to work. The SE load mentioned in RCA data sheets is 2k5, and I would suggest a 5k or 6k6 for the project. Take a look at the E-Linear schematic I stuck somewhere( Tubes maybe?), and figure putting the 'screen taps' at 10% or so for just little bit of NFB. If we use a 5687/6H6-Pi or the like, it won't be much.

As to naming it( when it is finally done...) let's leave Merlin for a really advanced revision or another circuit. To mate with the Guinevere linestage, I would be suggesting Arthur of course.
regards,
Douglas