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Hi Luis.
Yes, I tried it with a 500mm f4L Canon USM.
The results were very good, though there was a noticable lack of contrast compared to a very fine 5" doublet I had at the time.
It gave very fine wide-field views but ultimately not as good as a MUCH cheaper telescope that weighed 1/4 as much.
The 400 5.6 is a good, sharp lens but very aperture limited and with multi-lensed construction, I would count on fairly average contrast and you won't be seeing any mag 6 stars! IMO for this to be worthwile, you'd want at least a 200 2.8 or older FD 300 2.8, 400 2.8 or 5-600 f4.5.
Having said that, I would think the 400 f5.6 would be perfectly useable as a daytime scope.
I also found the contraption a little fiddly to make, but I'm not a good DIY'er either.
I may be wrong, but I think someone does make a similar device which I saw on an e-bay euro site. Sorry. I can't remember exactly where.
Unless you really wan't to double-up and/or can get one made or buy one cheapish, I think I'd leave it alone unless you can get a large aperture lens for little money.
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