If i had a camera i would show you how i did mine, but as i dont i'll try and explain.
I done it Pretty similar to hondour auctually, btw i use ordinary arcadia T8 reflectors couldnt find any for the Interpet PC T5 range and IMO they work very well.
First of all i worked on the reflectors, I got them at the best angle etc and then drilled a couple of small holes through them and then drilled straight through the hood, i then fixed on the reflectors with suitable screws to the hood.
The clip that came with the interpet tube i managed to push through the opening in the arcadia reflector (the opening is for the arcadia clip which will come with the reflector but it obviously wont fit the T5, so use the one supplied by interpet) i cant remember exactly how this worked but i was able to make it stay attached to the reflector (auctually i think i remember now, when i was attaching the reflector the clip went on first and made a sandwich between the reflector and the hood, and then i screwed it down, sorry my lights are on and i cant have a proper look)
So by following the above you now should have the reflector attached and the single clip supplied by interpet in place.
On the other end where the the ballast fitting meets the tube i got a cable tie and put this on the underside between the hood and the reflector and tied the light fitting in place and tightened it, i then inserted the tube in the light fitting and clipped it in place at the other end, job done.
Anyway i hope you can follow that, you will figure it out, the only thing i really had to do was drill a couple of holes through the reflector and the hood, the rest you will work out if you tie one end with a cable tie, the cable tie does not go on the tube only on the fitting at one end and the clip holds the other end.
Edit: btw the cable ties work just fine as long as they dont touch the tube, the tubes get to hot and would eventually melt them.