(excuse my poor english but i think that you will understand easily
The action of strings on Dobro, lap steel, Weissenborn is rather high, it is thus necessary to compensate for this height
They exists capo for this type of guitars but it is generally rather expensive
For a rare, occasional use, here is a sheap solution :
buy a nut of head, in bone rather at a luthier shop here in my case it was THOMANN
the nut cost me 5 euros.
to place it under strings in the place that you want to fret and to place upstream an old capo as here a Dunlop extended to make round of the neck, we can put so simply a pencil fixed around the neck by an elastic
Look please the pictures in joint pieces
(https://scontent.xx.fbcdn.net/v/l/t31.0-8/15800366_1213517928728544_2085606799403120877_o.jpg?oh=c5b3e3bf70a7fca2c7c0bb6a5ee8a594&oe=58DCC112)
(https://scontent.xx.fbcdn.net/v/t31.0-8/15844111_1213518418728495_8090976097320904596_o.jpg?oh=4e5eff4ca6b20db61d478b8c82db7db2&oe=58D97670)
(https://scontent.xx.fbcdn.net/v/t31.0-8/15896384_1213519138728423_7256190207396743818_o.jpg?oh=a3163dde13c9d135ec8a901427b2c8b6&oe=58D99518)
Clever. It's just like a nut riser for a round neck.
I'm quite drunk bit it still s3ems very ingenious.
Rock on!