Hi Himmel and welcome.
Firstly, I’m afraid your link doesn’t work for me, so I can’t comment directly upon your plan.
However, in general, I’m strongly against “Safari Rooms” for motorhomes. Motorhomes are normally very mobile (this is, of course, the beauty of them) but a Safari Room anchors them in one place – they’re a nuisance to erect and to take down every time you want to move the motorhome. Whilst they seem like a good idea, I would much prefer to have a cheaper and more flexible alternative – a tent. Pitched close to your motorhome, they serve the same purpose as a Safari Room, yet they don’t require taking down when you want to go shopping or drain the waste, or whatever. They also retain your pitch.
To modify a different-sized Safari Room to try and get it to fit your awning is not a good idea, in my opinion.
Best wishes,
Barry