Another Figured Bass problem

[Robert Blackstone]

Dear all, 

In the Lilypond documentation it is shown how to stack BC figures, but I have not been able to find how to put a figure in the second row with nothing above it.

In my score I get:

but what I need is:

I got this result by entering \  before the number, like < \  3+ >, and erasing the \ in the pdf.

Doable but maybe not worthy of LilyPond.

Is there a better method?

Thanks in advance for any advice.

Best regards,

Robert Blackstone