What about doing an Ugly Stik sort of test model for a smaller engine size? I'm thinking of a test model that copies the wing airfoil but allows you to try various slot arrangements. It could be powered by a 25 to 40 and the fuselage would be Wilga'ish but box shaped in section. The leading edge slots could be set up to either be adjustable or repaceable so that different add on plates similar to the Fiesler type could be used to position the slot airfoil at different locations.

The towplane is a pretty sizable chunk of money and time. Seems like a little sport model to test some of this stuff would be a good idea. As a bonus you could put the flaps on as well and it would make a pretty decent little sport model in it's own right afterwards I would think.