The board itself was about 800mm long x 250mm high (at the head end and 100mm high at the tail end) and around 25mm thick.  Arris or round all the edges.  Two holes were drilled, one in the tail end which was the fixing point to tie a nylon leader / trace with a lure on the end.  The other hole at the head end was around 20-25mm diameter.  A short length of dowel around 400mm long was shoved into this hole.  You want this to be a tight fit with about 25mm of the dowel protruding from the 'back side' of the reti.  Two short saw cuts were made where shown at the head end.  We would lock the dowel in place by tying it into position using cord.  The cord would be attached to the end of the dowel furthest away from the board then run through a saw cut, then looped around the short end of the dowel, back through the other saw cut and tied back tight to the long end of the dowel.

Attach your tow line to the end of the dowel and you're ready to go.  We would mostly run two lures each on a separate leader around 4m long.  One leader off the tail end of the board and another 3-4m up the tow line.  Lures were mostly hexagonal chrome slices but you could use anything really.  Saltwater flies / octopus skirts / soft baits would work better than metal slices when the current is slow.

The top /plan view would look like this.

This is what I did 50 years ago.  With today's tools and materials I'm sure there are better / simpler ways of attaching a wooden dowel or similar at right angles to a flat board.

The angler is standing at spot X and the river is flowing out.  The reti board will head out into the current at around 45 degrees.  By moving your position and letting more or less line out you can position the location of the lures where you want.

The line to the reti board is coloured green

The reti board is red

The leaders / traces to the lures are blue.

At one time I considered employing a similar system off my boat when game-fishing to give you an even wider spread of lures than you can achieve with outriggers.  I'm sure it would work in the right conditions however the increased risk of a  cluster f... especially with a small crew numbers put me off.  This short vid takes what I was thinking of to another level.

https://www.youtube.com/watch?v=UAk1BLR9g9Y  

Completely off topic, but I've done a bit of striped bass fishing in that part of the world. Forgot about umbrella rigs. They're bloody effective. Could be good for our game fish. Like a dredge with a hook at the end