You could get away without all those straps though (two for the thighs to keep it in place maybe an idea). Instead you'd have to have two leaf like springs back to back like this --> () with the dildo at the top knees either side, when you squeeze they'd flatten out and hey presto, thrusts ahoy! The more, erm, /travel/ you wanted the bigger those springs would have to be and the further out your knees would have to start.... It'd be one hell of a work out as well.
this is why engineers should never be allowed team up with scientists to design sex toys.