I do not think your going to be able to go to a bigger assembly. Mainly because there would be no way to extend past the current mounting point where the brackets are at.
Just by looking at the picture it appears you could squeeze another two more inches of roller tube up there and then slide the adjustment rod out another inch on each side.
Buying a bigger assembly would not be any benefit though as you are limited by slide wall length. IN your case all you would want is a slightly longer bare tube. You would then remove everything off your current roller tube and re-assemble ( end caps, torsion spring, etc. )
If you want to send me a picture showing the inside lip of the slide with the slide extended
I can better ascertain where your fabric and roller tube currently is as it pertains to the inside wall of the slide. You can e-mail them to me at firstname.lastname@example.org
For what it is worth, I have seen much worse slide coverage and shorter assemblies which challenges us when making new toppers. We have to really push the envelope on slide topper length when making new ones.
Let me know if you have other questions or want to bounce ideas off me and I will do what I can to assist you.