Werner:

Tip: Combine the elements of the top of your oven into a group. Then make the group invisible.
or
Tip: Alternatively export this part with 3D View --> Export to OBJ format and re-import it as a separate model.
or
Tip: Use the new feature Tools --> Copy as new furniture .

Tip: To make elements invisible use the Furniture list (the bottom left window). Search the element that should be invisible (CTRL-A selects all elements) and modify the visibility status in the edit window (double-click or right-click to open the menu).

Tip: Check the attachments list to find a round wall, as featured in your first link.

Tip: Order the furniture list by clicking one of the column headings. You can change which columns are visible by right-clicking and choosing Display column. Often I find it useful to see the Elevation column and sorting furniture by that column. That way the highest elements (the top of your oven) will cluster.

I hope this helps towards overcoming some of your problems.

Hans