The sun went to a very bad position from inside my home and so I can't really test my C5-PST setup. Until today, it seems not too bad, but real observation is still impossible, so I just try it despite part of the aperture is blocked.
I placed the Cheapy-ERF on the C5, and test the temperature at prime focus, it's not even warm, the Cheapy-ERF is very efficient to cut down the temperature of the focused solar disc. At least my C5 won't die due to the exccesive heat.
IR-UV has no guarantee here.
Second experiment was to test with a Herschel Wedge. Herschel Wedge cannot be used with compound telescope normally, since the unfiltered light collected and focused by the primary mirror, will kill the secondary mirror and the baffle tube. However, with the Cheapy-ERF in place, the above is proven safe.
I placed the Herschel Wedge there with the Cheapy-ERF, and then a ND3, I found it works perfectly. Of course, the optical quality will depend on the quality of the Cheapy-ERF, so whether it's better than a piece of Baader film is yet to be verified. However, at least it works and I can throw away my Baader film based filter.