xarax,
Thanks for the invitation to post to this subject. I'm afraid that I'd take the traditional process of applying two, adjacent square lashings. One between the vertical (with the vertical spar perpendicular to support surface) and bottom most horizontal spar; the other between the bottom most horizontal and 2nd horizontal spar with the 2nd spar positioned (supported) on top of the bottom horizontal spar. Can be done with one piece of line and there remains sufficient room between the spars for frapping the 2nd lashing.
The above assumes that additional supporting structure will provide lateral support to the vertical spar.
That said, where slick spars, heavy traffic, working due to wind (from experience), etc. are involved the lashing between the bottom-most horizontal spar and vertical spar tends to slip. One solution, when the vertical spar extends sufficiently beyond & above the two horizontal spar, is to provide suspension support to the bottom most horizontal spar by applying an Icicle Hitch to the vertical spar with the "load" end secured and tensioned to the bottom-most horizontal spar. A taut-line hitch can be used to provide adjustable tension.
The idea of using a constrictor knot, as suggested by one poster, around one of the spars instead of a clove hitch to start a lashing is a good one. In the above it would be the starting "knot" applied to the vertical spar, just below the bottom-most horizontal spar.
SaltyCracker
(formerly KnotInGuild... felt that name a bit glib.)