As for the “bump”, that has to do with the organ, so it can’t be used for the tabernacle.
Your comment about a “square” altar got me wondering. After a long search, I found a document from the USCCB titled “
Built of Living Stones: Art, Architecture, and Worship” The link points to where you can buy the document from the USCCB. It’s also available in PDF format at
catholic-doc.org/br/files/g-stones.pdf .
Paragraph 58 of the document states:
Although there is no specified size or shape for an altar, it should be in proportion to the church. The shape and size should reflect the nature of the altar as the place of sacrifice and the table around which Christ gathers the community to nourish them. In considering the dimensions of the altar, parishes will also want to insure that the other major furnishings in the sanctuary are in harmony and proportion to the altar. The mensa should be large enough to accommodate the priest celebrant, the deacon, and the acolytes who minister there and should be able to hold The Sacramentary [The Roman Missal] and the vessels with the bread and wine. Impact and focal quality are not only related to placement, size, or shape, but also especially to the quality of the altar’s design and worthiness of its construction. The altar should be centrally located in the sanctuary and the center of attention in the church.
(The “mensa” is the top of the altar and should be made of stone.)
So while a square altar may be canonically acceptable, the main question is if it would fit in with the rest of the church.