I'm planning on making a mounted beregond conversion, but I don't know how.
I'd rather not do the classic"cut-of-top-of-body-put-it-on-bottom half-of -cav-model", because I would have to buy another beregond model.
any other ways I could do it?

thanks

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"make me lunch... worthy of Mordor"

If you have a Knight of Minas Tirith (I'd personally try for the old metal one) and some patience with Green Stuff, you can just do a head-swap and sculpt on a cape, longer tabard and a bow & arrows setup. If you've got a plastic MT archer that you don't mind chopping up for parts, you can get those last even easier.
There are lots of articles available on adding a cape to a mini, and the extended tabard of the Citadel Guard is really the same principle, only easier.

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Smite the iron, shape the pewter, sculpt the putty.
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This way is really very effective... Also if you don't like many details then better for you...