From the advice above I tried the hose of my Lidl wet/dry vacuum, and yes it fits great over the exhaust port, but I found the back pressure from the hose to be too much and hampered the suction.
So I had a rummage around the garden, shed, and garage for bits and ideas.
Well, I've fashioned exhaust from what I had to hand. Namely a length of square 65mm down pipe and some 9mm ply off cuts.
Made a square, 300mm long ply manifold double inlet box that's closed at one end, this end is extended around 15mm, to rest on the lid, the other end is about 5mm beyond the rim edge, and a snug fit on the downpipe. Drilled two 68mm holes to match exhaust ports.
Then made a mitre on the down pipe, and glued up with super glue. This slides into the manifold box, and the vertical drop is 1550, to about 50mm off floor.
The sound reduction is fantastic and works with one or both motors on without need to do anything. And there is no back pressure, with one or both motors on , so suction is still great.
Come summer I might fit a diverter flap, which will allow me to exhast outside or inside the workshop.
Not put anything on bottom yet, as the warmth from the exhaust is great to warm the shop.
Will post photos tomorrow if of any interest.
Thanks for your inputs.