OK, so I searched the major brands and here is what will work as best as I can tell from the stated dimensions. Their is room for a 7.5" tube in the filter box, this is where it gets fuzzy as manufacturers state total length and their is almost universally a 1" long seal cap that is supported above the filter box. This listing covers any heater under 8.5" long and in the case of Sera they are 8.7" but the seal cap looks to be larger. This listing does not imply that I have personally tested it or endorse these heaters and the most we endorse is 75W, however many hobbyists have used 100W without issues.
1) Penn Plax Cascade, up to 75W should fit
2) Eheim Jager, up to 50W will fit, confirmed (oddly in Europe while the 75W sticks out a little the thermostat is lower so it works, the US version will not)
3) Sera, all sizes up to 100W should fit, Petco lists these and while Sera is not a huge player in the US they are a fairly respected brand in Europe, this would be one I would look at.
4) Hydor, 50W will fit.
5) Marineland/Aquarium Systems Visi Therm, as best as I can tell none will fit, but it is possible if the thermostat was lower in the tube they could work though they would stick up higher.
6) Hagen Marina, up to 100W should fit